1. Field of the Invention
The present invention relates to a three-dimensional image processing apparatus for obtaining three-dimensional information of an object from stereoscopic images captured by plural image capturing apparatuses and, more particularly, to a technique for adjusting directions of optical axes of the image capturing apparatuses.
2. Description of the Related Art
One of the methods of obtaining three-dimensional information of an object from images is a method of capturing images of a predetermined image capture area by plural image capturing apparatuses disposed at predetermined intervals and obtaining corresponding relations of image formation points of the object on the captured plural images among the image capturing apparatuses. This method is generally called a stereo image process.
In a three-dimensional image processing apparatus using the stereo image process, the optical axes of image capturing apparatuses are set to be parallel with each other, or the optical axes are set so as to cross each other. The latter case has an advantage such that a common visual field of the image capturing apparatuses is easily assured, and is effective, for example, when it is necessary to set the distance between image capturing apparatuses to be smaller than the width of the image capturing apparatus itself, and when the distance between the image capturing apparatus and an object is small. In addition, when the optical axes cross each other, there are advantages such that (1) estimation precision of a parameter describing geometrical arrangement between the image capturing apparatuses is stabilized, (2) since the parallax of an object projected in the vicinity of a center of an image becomes close to zero, the cost of parallax calculation can be reduced, and the like.
Although “crossing” has various advantages as described above, in reality, the optical axes are not seen, so that it is very difficult to adjust the directions of the optical axes of image capturing apparatuses. It is all the more difficult for the user having no expert knowledge of image processing.
The conventional technique has an attempt to perform a method of automatically adjusting an optical axis on the basis of correlation between center portions of images on the assumption that fixation points on the optical axes gather in the center of an image pickup device (that is, the optical axis passes the center of the image pickup device).
In the actual image capturing apparatus, however, there is no guarantee that the optical axis passes the center of the image pickup device due to mount variations of the image pickup device and an optical system (refer to FIG. 13). Moreover, there is also the possibility that a deviation amount between the passing point of the optical axis in the image pickup device (the optical center of the image) and the center of the image pickup device (the center of the image) changes when the focal distance changes. In the case where a center portion of an image does not include a characteristic image point and in the case where a similar pattern repeatedly appears in an image, erroneous correspondence between images may occur. Therefore, the precision of optical axis adjustment in the conventional technique is limited.